1. Technical Field
The present invention relates to a light-emitting device using light-emitting elements for emitting light having an intensity depending on the amount of current flowing from an anode to a cathode, such as an organic light-emitting diode element, to an image forming apparatus, and to a display apparatus.
2. Related Art
In recent years, a great deal of attention has been paid to organic electroluminescent elements or organic light-emitting diode (OLED) elements, which are also called light-emitting polymer elements, as next-generation light-emitting devices replacing liquid crystal devices. Since the OLED element is a self-emitting type, a panel using the OLED element has a low dependency on a viewing angle, and moreover, since the OLED element does not require a backlight or reflected light, this element is suitable for a thin display device with low power consumption.
The OLED element is a current driven element that has no voltage retention, like the liquid crystal element, and cannot maintain a light emission state when a supply of a current thereto is disconnected. Accordingly, when the OLED element is driven by an active matrix method, it is common that a driving signal to indicate a gray-scale level of a pixel is supplied to a pixel circuit via a data line, a voltage is written in a gate electrode of a driving transistor by supplying a driving signal to the gate electrode in a write period via a holding transistor in the pixel circuit, the voltage is retained by a gate capacitor and the like, and the driving transistor continues to cause a driving current corresponding to the voltage to flow into the OLED element.
In such a light-emitting device, the OLED element has an anode coupled to the driving transistor, and a cathode opposite to the anode. On the other hand, the cathode is formed opposite to the data line and the holding transistor (for example, see Japanese Unexamined Patent Application Publication No. 2003-316296 (see FIGS. 2 and 3)).
However, when the cathode is opposite to the data line, a stray capacitance is produced between the cathode and the data line. The stray capacitance acts as a capacitive load from the point of view of a driving circuit for driving the data line. Accordingly, if the driving circuit has low driving capability, there is a problem in that it is difficult to write a voltage signal into the data line at high speed, and the brightness of the OLED element is lowered. On the other hand, even if a driving circuit having high driving capability is used a high load still needs to be driven and therefore, there is a problem in that power consumption is increased. In addition, since the stray capacitance is increased when the holding transistor is opposite to the cathode of the OLED element, there is the same problem as in the data line. Particularly, when an operation state of the driving transistor is controlled by supplying a driving signal having a preset pulse width to the driving transistor and the brightness of the OLED element is controlled by the preset pulse width, there arises a problem in that the driving signal having the preset pulse width is deteriorated due to the capacitive load, and thus, the desired brightness may not be obtained.
More particularly, when the light-emitting device is applied to a head unit of an image forming apparatus, since the OLED element is required to have high brightness because of sensitivity of a photoreceptor, there arises a significant problem of lack of write due to the stray capacitance. In addition, if the write period in a large display panel is short, there is also a significant problem of lack of write due to the stray capacitance.